Journal of the Geological Society, London, Vol. 168, 2011, pp. 393–405. doi: 10.1144/0016-76492010-047.

393

3D seismic geomorphology and sedimentology of the Chalk Group, southern

Danish North Sea

STEFAN BACK 1*, HEIJN VAN GENT 2, LARS REUNING 1, J URGEN GROTSCH 3, JAN NIEDERAU 1

& PETER KUKLA 1

1Geological Institute, RWTH Aachen University, 52062 Aachen, Germany2Structural Geology–Tectonics–Geomechanics, RWTH Aachen University, 52056 Aachen, Germany

3Shell EP Europe, NAM, 9400 HH Assen, The Netherlands

*Corresponding author (e-mail: back@geol.rwth-aachen.de)

Abstract: Classically, the North Sea Chalk is interpreted as having been deposited under quiet, homogeneous

pelagic conditions with local redeposition in slumps and slides. Recent observations of highly discontinuous

reflection patterns on 2D and 3D seismic reflection data from the NW European Chalk Group have led to a

revision of some general ideas of chalk deposition, with the suggestion that long-lived, contour-parallel bottom

currents exerted a primary influence on the development of intra-chalk channels, drifts and mounds. This

study proposes an alternative explanation for the formation of selected intra-chalk seismic and stratal

discontinuities, interpreting these as being caused by gravity-driven processes that developed in response to

intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-

scale slumps, slides, debris flows and turbidites. The last occur in sinuous channel systems flanked by large

master levees, with the channel fill exhibiting well-developed secondary banks and overbanks on the outer

bends of the channel thalweg. This first documentation of channelized density-flow deposits in the North Sea

Chalk has important consequences for the interpretation and prediction of redeposited chalk units,

emphasizing at the same time the strength of detailed 3D seismic discontinuity detection for subsurface

sedimentary-systems analysis.

The Upper Cretaceous to Lower Palaeogene Chalk Group of NW

Europe is a biogenic deposit mainly of pelagic origin (e.g.

Hancock 1975; Kennedy 1987; Surlyk 1997; Surlyk et al. 2003,

2008; Van der Molen et al. 2005; Esmerode et al. 2008). Its

deposition is classically assumed to represent the settlement of

calcarous ooze from suspension, draping pre-existing submarine

morphology by a monotonous ‘blanket deposit’ with mainly

parallel bedding relationships and high lateral continuity (for a

critical discussion see, e.g. Evans et al. 2003; Surlyk et al.

2008). However, redeposited, less homogeneous chalk sediments

have also been identified in the past, mainly in and adjacent to

tectonically active areas (e.g. Hardman 1982; Brewster &

Dangerfield 1984; Hatton 1986; Bromley & Ekdale 1987; Cart-

wright 1989; Clausen & Huuse 1999; Skirius et al. 1999; Evans

et al. 2003; Lykke-Andersen & Surlyk 2004; Van der Molen et

al. 2005). Recently, the documentation of significant intra-chalk

erosional surfaces, channels, slump scarps and mass-transport

complexes on 2D and 3D seismic reflection datasets both

onshore (Evans et al. 2003) and offshore NW Europe (e.g.

Lykke-Andersen & Surlyk 2004; Van der Molen et al. 2005;

Esmerode et al. 2007, 2008; Surlyk & Lykke-Andersen 2007;

Surlyk et al. 2008) initiated a revision of the general ideas of

chalk deposition, with the most recent publications arguing that

powerful, long-lived, contour-parallel bottom currents acted as

the primary control for the development of intra-chalk channels,

drifts and mounds. This paper provides an alternative explanation

for the formation of many of the significant intra-chalk seismic

and stratal discontinuities by interpreting these as being caused

by gravity-driven erosional and sedimentary processes that devel-

oped in response to intense syndepositional tectonics.

The 3D seismic reflection survey analysed in this study covers

c. 2000 km2 of the Danish North Sea Central Graben, bound by

the Ringkøbing–Fyn High in the east, extending in its central

part across the Bo-Jens, Gorm-Lola, Tyra-Igor, Igor-Emma and

Adda Ridges, and reaching in the south the Danish Salt Dome

province (Fig. 1). The seismic reflection character of the Chalk

Group ranges from continuous to highly discontinuous. In the

case of pronounced seismic discontinuity, reflections exhibit

numerous truncations that result in isolated and oblique reflec-

tions with respect to the surrounding reflection packages, and

frequent lateral changes in reflection polarity indicating that

depositional units of differing impedance are juxtaposed against

each other. To gain a basinwide overview of the 3D distribution

of seismic discontinuity features throughout the Chalk Group, a

fast-track 3D horizon framework was constructed comprising

three iso-proportional surfaces between the Top- and Base-Chalk

marker horizons (Fig. 2; Top- and Base-Chalk interpretations

provided by Maersk Olie og Gas and DUC partners). This

subdivision defined three coarse interpretation levels, a basal

Chalk I level (Base-Chalk reflection to Iso-Surface 1), a central

Chalk II level (between Iso-Surfaces 2 and 3) and a topmost

Chalk III level (Iso-Surface 3 to Top-Chalk reflection; the prime

target interval for hydrocarbon exploration). In large parts of the

study area, the iso-proportional surfaces honour the general trend

of the subsurface stratigraphy; however, there are clearly regions

where they cut across seismic reflections, particularly in areas

characterized by pronounced stratal inhomogeneity (Fig. 2).

However, the use of such phantom horizons allows the reconnais-

sance analysis of extensive areas of highly discontinuous seismic

stratigraphy, whereas a reflection-based interpretation will result

in a 3D patchwork of countless, small horizon fragments that

prevents a broad, survey-wide seismic visualization. Hence, the

analysis of seismic reflectivity and discontinuity attributes

(chaos, variance) projected onto the iso-proportional surfaces

allowed a first-pass identification of seismically continuous areas

and regions characterized by subsurface reflection discontinuity

caused by features that were subsequently selected for detailed,

reflection-based seismic interpretation and 3D seismic–geomor-

phological analysis.

3D seismic overview based on iso-surface slicing

In the Chalk I interval between the Base-Chalk reflection and

Iso-Surface 1 (Fig. 2), seismic discontinuity data indicate that

most parts of the study area are characterized by coherent,

laterally continuous seismic reflections (Fig. 3a and b). This is

particularly the case at and immediately above the strong

amplitude peak interpreted as the Base-Chalk reflection (Figs 2

and 3c, d). Dissemblant reflection information at Base-Chalk

level mainly concentrates on the Bo-Jens and Igor-Emma Ridges

and the Coffee-Soil Fault Zone (Fig. 3a) where sharp, linear, in

places branching chaos-attribute signatures indicate the subsur-

face presence of faults. Stratal discontinuity is observed in the

northeasternmost of the study area in the hanging wall of the

Coffee-Soil Fault, where slightly mounded reflections with a

lobate termination occur. These features are suggestive of

isolated, small-scale mass-transport complexes (Fig. 3d). In the

southernmost part of the study area, a pair of sharp, parallel-

trending, slightly sinuous to almost linear chaos-attribute features

that extend in a NW–SE direction over a length of c. 10 km

(Fig. 3b and c) indicate the presence of a c. 1 km wide incision

immediately west of the southernmost salt-rim syncline. This

discontinuity feature can be interpreted as a weakly incised low-

sinuosity channel or channel segment (Channel I; Fig. 3b and c).

Another subtle, almost linear reflection irregularity descends over

20 km from the Tyra-Igor Ridge in the central part of the study

area towards the southern Salt Dome province (Fig. 3a and b).

This elongate seismic discontinuity is also characterized by a

Fig. 1. Regional map showing the location of the study area in the North

Sea Central Graben offshore Denmark and the principal tectonic

elements of the Central Graben area (modified after Cartwright 1991;

Vejbæk & Andersen 2002; Japsen et al. 2003; Esmerode et al. 2008).

The location of the 3D seismic dataset analysed in this study is indicated

by the thick black outline. The black horizontal line indicates the position

of the seismic line shown in Figure 2.

Fig. 2. Illustration of the initial, fast-track 3D seismic interpretation approach used in this study. This approach is based on the calculation of three iso-

proportional surfaces (phantom horizons) between the Top-Chalk (negative) and Base-Chalk (positive) marker reflections, subdividing the Chalk Group

into three preliminary interpretation levels, a Chalk I level (Base-Chalk reflection to Iso-Surface 1), a Chalk II level (between Iso-Surfaces 2 and 3) and a

Chalk III level (Isosurface 3 to Top-Chalk reflection). Lateral seismic visualization along and between the iso-surfaces is survey-wide. This approach

allows identification of seismic discontinuity features (Figs 3–5), which can be subsequently mapped in detail with reflection-based seismic interpretation

techniques honouring stratigraphic relationships.

S . BACK ET AL .394

Fig. 3. Chalk I overview interpretation. (a) Chaos-attribute data extracted in Chalk I interval, and (b) survey-wide interpretation. The discontinuity-slice

data show that at Chalk I level vast areas are characterized by highly continuous stratal configuration (white areas; laterally discontinuous seismic

signature shown in black). (c) Close-up view of lateral chaos-attribute extraction in the south of the study area (upper left), synoptic display of lateral

reflectivity distribution (upper right; reflectivity data in ‘European’ zero-phase polarity; positive amplitudes in blue, negative amplitudes in red, which

applies for all lateral and vertical reflectivity displays presented in the following figures), and vertical reflectivity section (bottom) of the slightly sinuous

Channel I (length c. 10 km, width c. 1 km). Yellow crosses on reflectivity image indicate the base of the channel incision. (d) Variance-attribute close-up

(top) and vertical reflectivity display (bottom) of a seismic discontinuity feature observed along the Coffee-Soil Fault in the northeasternmost part of the

study area, where slightly mounded reflections with lobate terminations suggest the occurrence of small-scale mass-transport deposits.

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 395

low degree of sinuosity, little basal incision, and a thickness of

less than 20 ms (two-way travel time; TWT). The average width

of the discontinuity feature is around 200 m, with local dissem-

blance bulges in its central part up to 500 m wide (Fig. 3a and

b). This feature can also be interpreted as low-sinuosity channel

(Channel II), with the central dissemblance bulges suggestive of

crevasse splays (Fig. 3a and b). The system terminates in a flat-

topped, triangle-shaped lateral amplitude irregularity (additional

positive-to-negative seismic loop) of c. 10 km2 extent in the

southernmost salt-rim syncline of the study area (Fig. 3a and b).

Its shape is indicative of a frontal splay deposit sensu Posamen-

tier & Kolla (2003), with its reflection signature suggesting a

deposit of lower density than the surrounding chalk.

In the Chalk II interval (Iso-Surfaces 1–3; see Fig. 2), large

parts of the survey area are characterized by reflections produ-

cing a highly discontinuous, rather chaotic seismic pattern (Fig.

4). The surface-slice analysis of chaos-attribute discontinuity

data (Fig. 4a) reveals that subparallel, continuous reflection

patterns are generally rare; instead, a multitude of reflection

discontinuities occur, including seismic reflection patterns that

are suggestive of large-scale mass-transport systems (slides and

slumps; Fig. 4b). There are also a variety of linear and curved

dissemblance features resembling channel systems that are

oriented in different directions, including those with trends

mainly southward (Channels III and IV on the eastern flank Bo-

Jens Ridge; Fig. 4b–d), to the east (Channel V on the north-

eastern flank Tyra-Igor Ridge; Fig. 4b), and westward (Channel

VI in the northern part of Salt Dome province; Fig. 4b). The

vertical display of reflectivity data also shows a pronounced

seismic discontinuity in the central Chalk II interval with

numerous reflection terminations at steeply dipping, U- and V-

shaped truncation surfaces interpreted as channel incisions (Fig.

4d). Within these incisions, asymmetric reflection patterns, a

lateral and vertical migration of erosion surfaces, and the

truncation of older incision or infill reflections by younger

erosional features are common, as is the local preservation of

onlapping incision fill suggestive of channel inner banks and

overbank units (Fig. 4d). The sidewalls of the major incision

features are locally influenced by slope instability, as indicated

by the occurrence of small-scale faults (linear chaos-attribute

discontinuities) trending parallel to the channel axis that are

associated with small, arcuate-shaped discontinuities interpreted

as slides or slumps (e.g. western flank of Tyra-Igor Ridge, Fig.

4a and b). On the southwestern and northeastern flanks of the

Tyra-Igor Ridge, more extensive (.150 km2), arcuate zones of

highly discontinuous seismic facies are interpreted to record the

subsurface presence of large-scale mass-transport complexes

(MTC; Fig. 4a and b). Vertical reflectivity sections across these

features show reflection units of significant internal distortion

that descend from the Tyra-Igor Ridge both into southwestern

direction into the Salt Dome province (MTC A; Fig. 4e), and

towards the NE into the Coffee-Soil fault zone (MTC B; Fig. 4f).

The observation of U-shaped erosive features above these

distorted units (Fig. 4e and f) suggests that sliding and slumping

was succeeded by channel incision.

In the Chalk III interval between Iso-surface 3 and the Top-

Chalk reflection (Fig. 2), the survey-wide seismic chaos-attribute

data documents that the reflection pattern is again rather homo-

geneous and of relatively high lateral continuity (Fig. 5a and b),

similar to the Chalk I interval. In contrast to the multitude of

linear discontinuity features observed in the Chalk II interpreta-

tion level below, the presence of a horizontally dendritic to

circular (Fig. 5a and c), vertically mounded seismic discontinuity

facies (Fig. 5d) characterizes this interval, primarily on the Adda

and Igor-Emma Ridges. Outside these areas, minor linear seismic

discontinuity features interpreted as faults characterize the Gorm-

Lola Ridge and the western survey boundary (Fig. 5a and b).

The seismic overview analysis based on the fast-track, iso-

proportional slicing of the Chalk Group interval in the study area

documents that the occurrence of seismic discontinuity facies is

almost as common as seismic reflection continuity, which is in

line with previous studies that identified significant intra-chalk

stratal inhomogeneity (e.g. Evans et al. 2003; Lykke-Andersen &

Surlyk 2004; Van der Molen et al. 2005; Esmerode et al. 2007,

2008; Surlyk & Lykke-Andersen 2007; Surlyk et al. 2008). At the

Chalk II level (Fig. 4), stratal discontinuity seems dominant with

numerous reflection truncations, incisions and the presence of

extensive reflection packages of significant internal distortion,

features that are indicative of considerable erosion, transport and

redeposition of chalk material. To evaluate the validity of the

first-pass interpretations based on iso-proportional seismic

volume slicing, the following paragraphs provide a more detailed,

horizon-based analysis of the morphology and seismic expression

of the most prominent reflection discontinuity features identified

in the study area, focusing on (1) the linear and sinuous incision

features suggestive of channels, (2) the internally distorted

reflection packages suggestive of slide, slump and possibly

debris-flow deposits, and (3) the pronounced seismic discontinuity

facies that characterizes considerable areas at the Chalk III level.

Detailed seismic interpretation and analysis of keystratal discontinuities

Seismic incision features interpreted as channels

Both the north and south of the study area exhibit extensive

zones of seismic reflection truncation by elongate discontinuity

features interpreted as channels. In the northern part of the study

area, the most intensive zone of probable channel incision is

located between the Bo-Jens and Adda Ridges (Fig. 4; also see

Esmerode et al., 2008). With a width of over 10 km in its

proximal part, the main incision pathway can be traced for c.

20 km into a region west of the Gorm-Lola Ridge. In the

southern part of the study area, the main location of reflection

truncation by elongate dissemblance features is located east of

the Gorm-Lola Ridge (Fig. 4). The largest incision features

observed in this part trend in a westerly direction on the

southwestern flank of Tyra-Igor Ridge, turning southward where

entering the salt-rim syncline east of the Gorm-Lola Ridge.

Figure 6 illustrates the results of a detailed, reflection-based

interpretation of the proximal regions of one main U-shaped

incision between the Bo-Jens, Tyra-Igor and Adda Ridges inter-

preted as Channel IV (also see Fig. 4b–d). Reflection picks that

define the outline of the studied system were placed at the

negative-to-positive zero-crossing at the incision base and flanks,

coinciding in many places with prominent terminations of the

surrounding reflections (see Fig. 6, vertical reflectivity data).

Within the incision, lap surfaces and reflection truncations were

mapped to highlight infill variations along its course. Channel IV

has an average width of 2.5 km, with an erosional base cutting

along a sinuous downslope trend several tens of ms TWT into the

underlying substratum. Gull-wing shaped reflections at the outer

edges of the incision are interpreted as master levees that flank the

channel in its northern, proximal part on both sides (Fig. 6,

sections A–A’ to C–C’). These features are less clear in the south

(Fig. 6, sections D–D’ and E–E’) where a tributary channel

(Channel III, see Fig. 4c and d) enters Channel IV. Within the

incision, various small-scale reflection packages either onlap the

S. BACK ET AL .396

Fig. 4. Chalk II overview interpretation. (a) Chaos-attribute data extracted in Chalk II interval, documenting that most of the survey area is characterized

by discontinuous stratal configuration (grey to black areas). (b) Interpretation of discontinuity-slice data. Elongate linear to sinuous incisions interpreted as

channels in blue, mass-transport complexes in red. Blue arrows show downstream direction of channels as indicated by dendritic channel patterns

upstream; red arrows indicate mass-transport direction indicated by upslope headwall scarps and/or downslope folds and thrusts. (c) Close-up of sinuous

chaos-attribute pattern suggestive of intra-chalk channels between the Adda and Bo-Jens Ridges (location of close-up indicated in (a)). (d) Vertical

reflectivity images along sections W–W’ and X–X’ (location indicated in (c)) showing U- and V-shaped incisions interpreted as Channels III and IV.

(Note reflection pattern within Channel IV indicative of inner-levee deposits; for a detailed interpretation see Figs 6 and 7). (e) SW–NE-oriented mass-

transport complex with major internal folding located on the southern Tyra-Igor Ridge along section Y–Y’ (indicated in (b)). Yellow crosses indicate

incision feature above mass-transport complex. (f) ENE–WSW-oriented mass-transport complex on the eastern flank of the Tyra-Igor Ridge along section

Z–Z’ (location indicated in (b)). Yellow crosses as in (e).

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 397

sidewalls or downlap onto the incision base (Fig. 6, sections A–A’

to D–D’). These units occur dominantly at the outer bends of the

channel thalweg, and are interpreted as inner-levee deposits (or

secondary banks and overbanks sensu Kolla et al. 2007). The

variable nature of the infill is also indicated by the numerous

reflection truncations that can be observed along the main channel

axis in a downslope direction (Fig. 6, Section F–F’).

Figure 7 illustrates the lateral variability of the channel infill

through time with a series of successive reflectivity surface slices

at given distances above the channel base. At the lowest

stratigraphic level (20 ms TWT above the main erosional base;

Fig. 7a, top), bow-shaped, subparallel reflections of variable

polarity characterize the outer bends of Channel IV. These

features correspond to the small-scale onlap and downlap units

interpreted in vertical section as inner-levee deposits (Fig. 6).

Polarity-based reflection tracing in a downslope direction reveals

a laterally migrating line pattern (Fig. 7a, bottom) suggestive of

a shift of the channel thalweg towards the channel centre with

time. Opposite the inflow point of Channel III, lobate reflections

of strong negative amplitude are interpreted as a small-scale

slump deposit that entered Channel IV from the eastern master

levee. Up section, 40 ms TWT above the incision base (Fig. 7b,

top), bow-shaped outer-bend reflections in the northern part of

the system are mainly the topward continuation of those of the

preceding interpretation level; however, they are of different

orientation in the central and southern part of Channel IV.

Downslope reflection tracing suggests that thalweg sinuosity,

migration and inner-levee development were concentrated at this

interpretation level on the central and southern part of the system

(Fig. 7b, bottom). At 60 ms TWT above the main erosional base

of Channel IV (Fig. 7c, top), all remaining outer-bend reflections

correspond to those of the previous interpretation levels (Fig. 7c,

bottom). Downslope reflection tracing indicates a broad, straight

channel thalweg flanked by elongate reflections of relatively high

lateral continuity (Fig. 7c, top). These reflections extend top-

wards beyond the confines of the master incision and overlie the

master levees (corresponding to the overspill units, Fig. 6). This

suggests that Channel IV was filled by this time and that

channelized flows could no longer be contained.

In comparison with the northern part of the study area, intra-

Fig. 5. Chalk III overview interpretation. (a) Chaos-attribute data extracted in Chalk III interval, and (b) interpretation. The discontinuity-slice data show a

general decrease of seismic-discontinuity facies when compared with the preceding Chalk II interval (Fig. 4). Hatched areas on Igor-Emma, Bo-Jens and

Adda Ridges correspond to the dendritic to circular seismic discontinuity facies characterizing considerable parts of the Chalk III interpretation level. (c)

Close-up of the dendritic to circular, vertically mounded seismic discontinuity facies (chaos attribute; location of close-up indicated in (a)). (d) Vertical

reflectivity section along section X–X’ (location indicated in (c)), exhibiting at Top-Chalk level numerous small-scale mounds and depressions, and

frequent polarity changes in the reflections below.

S. BACK ET AL .398

chalk channels in the south are less incised and thus more

difficult to detect. Figure 8 is a representative example of a

southern incision of c. 1 km width that descends the south-

western flank of Tyra-Igor Ridge in a westerly direction. This

incision apparently truncates reflections a few kilometres north

of a salt structure, before turning in a SSE direction whereupon

it enters the salt-rim syncline east of the Gorm-Lola Ridge (see

Fig. 4b, Channel VI). The geomorphology and internal seismic

character of this incision are fundamentally different from those

of the northern system described above. The southern example

lacks sinuosity and is strongly asymmetrical. Its southern flank is

generally steep and erosive, whereas its northern flank has a

lower dip and is only weakly confined to partly unconfined.

There are no outer levees flanking the incision, and there is no

indication of inner levees within the incision fill. Instead, the

infill gives rise to a semi-transparent seismic facies comprising

weak, but distinguishable low-amplitude reflections. These in-

dicate the presence of well-layered depositional units, which are

difficult to detect seismically if their polarity matches that of the

surrounding strata.

Distorted reflection packages suggestive of mass-transportcomplexes

A key observation in the study area is that the majority of

large-scale, arcuate, internally distorted seismic units interpreted

as gravity-driven mass-transport complexes are located on, or in

the immediate vicinity of the flanks of rising structures (inver-

Fig. 6. Detailed interpretation of the proximal part of Channel IV between the Bo-Jens, Tyra-Igor and Adda Ridges (for location see Fig. 4a and c).

Reflection picks defining the channel outline are indicated on vertical reflectivity sections by yellow crosses. Channel IV has an average width of 2.5 km,

with an erosional base cutting along a sinuous downslope trend several tens of milliseconds TWT into the underlying substratum. Prominent master levees

flank the channel in its northern, proximal part on both sides (sections A–A’ to C–C’), but are less clearly defined in the south (sections D–D’ and E–

E’). In places, inner levees (secondary banks and overbanks sensu Kolla et al. 2007) are developed within the infill of the master incision (sections A–A’

to D–D’), features that dominantly occur on the outer bends of the channel thalweg. The variable nature of the incision infill is also indicated by

numerous reflection truncations observed along the main channel axis in a downslope direction (section F–F’).

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 399

Fig. 7. Projection of reflectivity surface slices at given distances above the base of Channel IV (top), and interpretation of channel thalweg development

(bottom; dashed lines indicate abandoned pathways). (a) Surface slice located 20 ms TWT above the channel base exhibiting bow-shaped, subparallel

reflections of variable polarity interpreted as inner-levee deposits at the outer bends of Channel IV. Polarity-based reflection tracing in downslope direction

suggests a shift of the channel thalweg towards the channel centre with time. (b) Surface slice located 40 ms TWT above the channel base. Outer-bend

reflections in the north are a topward continuation of those of the preceding interpretation level, but of different orientation in the centre and south.

Downslope reflection tracing indicates that thalweg sinuosity, migration and inner-levee development concentrated on the central and southern part of the

system. (c) Surface slice located 60 ms TWT above the channel base. Remaining outer-bend reflections in the NE correspond to those of the previous

interpretation levels. Downslope reflection tracing indicates a broad, straight channel thalweg flanked by elongate reflections of relatively high lateral

continuity. These reflections extend topwards beyond the confines of the master incision (see overspill reflections, Fig. 6). This suggests that Channel IV was

filled by this time and that channelized flows could no longer be contained.

S . BACK ET AL .400

sion ridges, diapirs). These features can be very diverse in

external form, size and internal architecture (e.g. Figs. 3d and

4e, f). Probably the most obvious and, with an extent of around

150 km2, also the largest seismic discontinuity unit is observed

in the lower Chalk II interval east of a series of linked, cuspate

chaos-attribute features resembling major headwall scarps (sensu

Bull et al. 2009) between the Tyra-Igor and Igor-Emma Ridges

(MTC B; Fig. 4a, b and f). Although close to the limit of

seismic resolution, vertical displays across this discontinuity

unit indicate the presence of irregular reflection contacts

associated with high lateral and vertical amplitude variations. In

other settings, similar features have been interpreted as slump

deposits (e.g. Moscardelli & Wood 2008). The second largest,

arcuate seismic discontinuity unit of the study area is located at

the same stratigraphical level on the southwestern flank of Tyra-

Igor Ridge (MTC A; Fig. 4a, b and e), covering an area of

around 100 km2 (Fig. 9). In its distal parts, this unit exhibits

trains of wavy reflection distortions associated with prominent

lateral amplitude irregularities suggestive of intense stratal

folding (and possibly thrusting) in a compression-ridge setting.

In comparison with the large-scale mass-transport complex on

the eastern side of Tyra-Igor Ridge (MTC B; Fig. 4f), this

deposit lacks indications of a sharp, well-defined headwall scarp

in its most proximal portion (MTC A; Fig. 4a and b). However,

the considerable reflection distortion in its distal part also

suggests its origin as a slump, although the relatively undis-

turbed reflection pattern in its most proximal portion (Fig. 9a

and b) indicates (for this part of the system) only minor

downslope sliding.

Other seismic discontinuity packages interpreted as mass-

transport complexes include the small-scale, mounded dissem-

blant units (,20 km2 extent) documented in the Chalk I interval

in the northeasternmost part of the study area on the hanging

wall of the Coffee-Soil Fault (Fig. 3a, b and d). These units are

generally less than 30 ms (TWT) thick and characterized by

lateral reflection pinch-outs, lobate frontal terminations and an

irregular top commonly marked by downslope-oriented ridges

and scour-like features (Fig. 3d). These small-scale deposits are

Fig. 8. Channel VI in the south of the study

area descending the southwestern flank of

Tyra-Igor Ridge in a westerly direction (for

approximate location see Fig. 4b). This c.

1 km wide, strongly asymmetric incision

feature exhibits a steep, erosive southern

flank and a gentle northern flank that is

only weakly confined (partly unconfined).

The system lacks any sinuosity and

evidence for levees, and is filled by

subparallel- to parallel-bedded,

homogeneous sheet deposits, features that

are suggestive of a bottom-current

controlled contourite system.

Fig. 9. Depth map of the top of the second

largest mass-transport complex in the study

area, located on the SW flank of Tyra-Igor

Ridge. Vertical reflectivity sections A–A’

and B–B’ reveal prominent high-amplitude

reflections suggesting distortion of bedded

material, which are interpreted as

compressional ridges within the mass-

transport complex. (For cross-section Y–Y’

see Fig. 4e.) Horizon picks defining the top

of the mass-transport system are indicated

on the reflectivity sections by yellow

crosses.

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 401

interpreted either as small-scale slump units or as debris-flow

deposits derived from unstable slopes further west.

Seismic discontinuity facies at Chalk III level

A seismic discontinuity facies associated with a generally

rugged, mounded reflection morphology characterizes the inter-

pretation level immediately below and at Top Chalk particularly

on the Adda and Igor-Emma Ridges (Figs 5 and 10a). Vertical

sections across the main zone of dissemblance (Figs 5d and 10b)

exhibit at Top-Chalk level numerous small-scale mounds and

depressions with a width of 100–300 m, a structural relief of up

to 25 ms (TWT), and frequent polarity changes in the strong,

highly discontinuous reflections below. Huuse (1999) has docu-

mented Top-Chalk features of comparable shape and dimension

on 2D high-resolution seismic reflection data in the Norwegian–

Danish Basin, interpreting these as possible surface karst struc-

tures. A difference between the features analysed by Huuse

(1999) and those of this study is the major reflection disruption

and lateral polarity variation in the reflection packages below the

Top-Chalk level (Figs 5d and 10b), in contrast to the rather

continuous, homogeneous internal reflection signature of the

Norwegian–Danish Basin features.

A comparison of the lateral distribution of the Top-Chalk

discontinuity facies in the study area with the seismic dissem-

blance features of the preceding Chalk II level (see Fig. 4)

indicates that the location of discontinuous Chalk III reflections

approximately corresponds to the sites of former mass wasting,

although much larger in area. To test whether the Chalk III

dissemblance facies could also be related to gravity-driven

processes, as opposed to an arguable interpretation as surface

karst (Huuse 1999) or possibly hypogenic subsurface karst (e.g.

sensu Palmer 1991), the approximate palaeo-relief at Chalk III

times was restored by flattening an auxiliary intra-Tertiary

horizon immediately above Top Chalk (Fig. 10b and c). Although

simple, this method seems appropriate in a setting characterized

at Chalk II level by a depositional thinning onto former structural

highs (e.g. Figs 4, 9 and 10; palaeo-highs interpretable as such if

exhibiting large-scale mass-transport systems on their flanks).

The flattening approach resulted in an eastward inclination (c.

1–28) of the Chalk III unit suggesting that the discontinuity

facies might have indeed formed on a palaeo-slope, an area that

could have been post-depositionally uplifted and tilted to its

Fig. 10. Seismic discontinuity facies associated with a generally rugged morphology immediately below and at Top-Chalk level. (a) Landmark structure

attribute map, corresponding in location to Figure 5c. (b) Unflattened vertical reflectivity display along section X–X’ (section location indicated in (a)).

(c) Vertical reflectivity section flattened on auxiliary horizon above Top Chalk. (d) Interpretation of the Chalk III discontinuity facies to have formed by

gravity-sliding in a palaeo-slope setting.

S . BACK ET AL .402

present-day position at the crest of the Tyra-Igor Ridge. In the

context of the multitude of gravity-driven deposits documented

in the preceding Chalk II interval, it seems possible that during

Chalk III times a slope setting facilitated gravity-driven down-

slope movement (slumping, sliding or creep?) contributing to the

irregular Chalk III morphology and seismic facies (Fig. 10d).

However, considering the complexity of tectonic inversion in

time and space compared with the simplicity of the seismic-

flattening approach, other interpretations of the seismic disconti-

nuity facies at Chalk III level are not invalidated, and further

studies including borehole-core data are needed to determine the

exact nature of the dissemblant Top-Chalk unit.

Discussion

The seismic reflection data and interpretations presented in this

paper suggest strongly that the North Sea Chalk in the Central

Graben offshore Denmark contains a multitude of mass-transport

deposits. The majority of these are located on, or in the

immediate vicinity of tectonically active structures, in particular

on the flanks of ridges that formed in response to Late Cretac-

eous inversion (e.g. Vejbæk & Andersen 1987, 2002; Esmerode

et al. 2008). The most obvious mass-transport units are the large-

scale slumps and slides on the flanks of the Tyra-Igor and Igor-

Emma Ridges (e.g. Figs 4, 9 and 10); more subtle features are

small-scale slump or debris-flow deposits in the vicinity of the

Coffee-Soil Fault (e.g. Fig. 3d). Stratal thinning towards former

structural highs suggests that horizon flattening can be used as a

first-pass indicator for palaeo-slope settings (e.g. Fig. 10), but the

lack of well-defined palaeo-bathymetric reference data for the

Chalk Group limits a more detailed 3D restoration of the palaeo-

morphology (e.g. sensu Back et al. 2008). However, the wide-

spread inhomogeneity of seismic facies still documents that

much of the Chalk Group material in the Central Graben was

remobilized after deposition and transported by sliding or slump-

ing, or as channelized density flow (Figs 3–5). This clearly

contrasts with the classical view of a highly continuous, dom-

inantly parallel-bedded North Sea Chalk, but adds to studies that

previously identified considerable intra-chalk erosion, sediment

reworking and redeposition (e.g. Quine & Bosence 1991; Evans

et al. 2003; Lykke-Andersen & Surlyk 2004; Van der Molen et

al. 2005; Jarvis 2006; Esmerode et al. 2007, 2008; Surlyk &

Lykke-Andersen 2007; Surlyk et al. 2008).

A significant part of the very recent work on dynamic

sedimentation processes during the development of the North

Sea Chalk (e.g. Esmerode et al. 2007, 2008; Surlyk & Lykke-

Andersen 2007; Surlyk et al. 2008) has stressed the influence of

powerful, long-lived, contour-parallel bottom currents for the

development of many of the intra-chalk stratal irregularities, an

interpretation particularly reliant on the analysis of channel

geometries (see Esmerode et al. 2008; Surlyk et al. 2008; and

discussions therein). However, the quantity, extent, spatial dis-

tribution, variability in transport direction and seismic character

of unambiguously gravity-driven mass-transport complexes (i.e.

slides, slumps) documented in this study suggests a critical re-

evaluation of the main controlling factors for the deposition of

the North Sea Chalk of the Central Graben. With gravity-driven

sediment transport seemingly common, it appears likely that

turbid density flows contributed to the redeposition of chalk

strata. Such currents that could have been generated, for

example, from the dispersed head-parts of debris flows (Nemec

1990), or simply evolved from such flows whenever these

accelerated sufficiently to become fully turbulent (Hampton

1972; Ferentinos et al. 1988; Weirich 1989; Nemec 1990).

Following this line of argument, it is significant that the

orientation of the interpreted channel systems varies throughout

the study area, with the channels of the Chalk I interval trending

into southwesterly and southeasterly directions (Fig. 3), and the

channels of the Chalk II interval being oriented towards the

south, SE, east and west (Fig. 4). No channels were detected in

the Chalk III level. All channels of the Chalk I and Chalk II

interval trend approximately perpendicular to the palaeo-slope

(see e.g. channel base of section F–F’, Fig. 6, in relation to the

Top Chalk reflection); however, its has been noted before that a

palaeo-slope restoration in the Chalk is difficult in the absence of

a clear palaeo-bathymetric datum. The most prominent channel

system of the study area is the major southward-trending

Channel IV at Chalk II level shown in Figures 4, 6 and 7, which

is partly the same system as analysed and described by Esmerode

et al. (2008). The channel is branched in its upper part, with

tributaries descending from the Bo-Jens Ridge in the west and

the Adda Ridge in the NE, before combining into one large

channel that follows the course of the structurally defined

depression between the Bo-Jens and Tyra-Igor Ridges (Figs 2

and 4). A key departure from the interpretation of Esmerode et

al. (2008) is the documentation of a clear sinuosity of the main

Channel IV incision that is flanked on both sides by master

levees (Fig. 6), which suggests its exposure to (and possibly

initiation by) turbulent erosive flows. In addition, the infill of the

system comprises well-defined inner levees on the outer bends of

the channel thalweg (see Figs 6 and 7), which strongly indicates

that the master incision served after its initiation recurrently as a

conduit for turbulent density flows. Towards the top of the

incision, secondary overbanks extend beyond the confines of the

master channel covering the master levees (Figs 6b, c and 7c).

This is interpreted to reflect that most of the channel was filled

by this time and channelized flows could no longer be contained.

The starting points of the system on the flanks of Bo-Jens and

Adda-Ridges (Fig. 4) indicate that it might have been fed initially

by slumped material from the actively rising and locally failing

ridge slopes. Downslope, it was further supported by mass-

wasted material derived from channel-wall collapses. The appar-

ently strong incision of the feature suggests a considerable

erosive power of the system, possibly enhanced by the incorpora-

tion of failed chalk hardgrounds in density flows passing through

the channel. Thus, the northern channel system of the Chalk II

interval displays many of the architectural elements of a classic

gravity-driven deep-water turbidite system (e.g. sensu Deptuck et

al. 2003, 2007; Posamentier & Kolla 2003; Heinio & Davies

2007; Kolla et al. 2007), features that have, to our knowledge,

not yet been documented in the Chalk Group or in other

carbonate depositional environments.

In comparison with the northern system, the channels in the

south of the study area (e.g. Figs 4 and 8) are less confined and

less incised. An important discussion point is that the southern

Channel VI (Fig. 8) initially trends in a westerly direction (see

also Fig. 4), which is oblique to the general southeasterly

direction of the long-lived, contour-parallel bottom currents

proposed by Esmerode et al. (2008) and Surlyk et al. (2008).

However, after passing the northern flank of a salt structure, the

system turns to a SSE direction (see Fig. 4b), a flow direction

that is in line with the proposed bottom-current orientation of

Esmerode et al. (2008) and Surlyk et al. (2008). From seismic

data alone it can only be speculated that increased chalk

cementation around the salt structure acting together with a

syndepositional rise of the diapir may have constrained the

location of Channel VI, and that, in turn, a positive diapir relief

could have caused a local deflection of originally SSE-oriented

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 403

bottom currents into a westerly direction. However, what is

clearly documented by the seismic reflection data is that Channel

VI generally lacks any sinuosity and evidence for levees, and

that the system is strongly asymmetrical and filled by subpar-

allel- to parallel-bedded, homogeneous sheet deposits. These

observations contradict an interpretation of the southern channel

as a turbidity-current feature; instead, they support its interpreta-

tion as a channel formed by bottom currents filled in laterally by

drift deposits.

In summary, the observation of significant stratal discontinuity

in the Chalk Group of the Danish part of the North Sea Central

Graben indicates that chalk sedimentation was strongly influ-

enced by submarine erosion, sediment transport and redeposition.

The mass-transport systems analysed in this study are only

mildly developed in the Chalk I interval, but reach a maximum

in the Chalk II and lower Chalk III levels (Fig. 11). In particular,

the major Chalk II slumps and slides that are located alongside

tectonically active inversion structures indicate that tectonically

induced, gravity-driven mass-transport processes contributed sig-

nificantly to the redistribution of chalk sediment. Additionally,

detailed analysis of the geomorphology and internal architecture

of the northern intra-chalk Channel IV (Figs 4, 6 and 7) suggests

that this system formed an important conduit for recurrent,

gravity-driven turbulent flows that transported failed chalk

material into the deeper chalk depocentres. However, at the same

stratigraphic level there are also incision features similar to

channels formed by bottom currents that have been documented

and analysed previously in the Chalk Group by, for example,

Surlyk & Lykke-Andersen (2007), Esmerode et al. (2008) and

Surlyk et al. (2008). Thus, it seems that a complex interaction of

inversion tectonics, gravity-driven processes and superimposed

bottom currents ultimately controlled chalk reworking and

redeposition in the study area, and that redeposited chalk can

consequently occur in a highly variable depositional fashion at

many locations in the North Sea Central Graben. Inferences on

whether the various reworked (allochthonous) chalk units have

reservoir properties different from autochthonous chalk deposits

are difficult to make from the use of seismic reflection data

alone, but the delineation of key study sites for integrating the

seismic interpretation results with high-resolution wireline-log

and core data is possible. Such a study has the potential to

identify unconventional hydrocarbon traps in previously unex-

plored chalk layers.

Conclusions

(1) The development of the Chalk Group of the North Sea

Central Graben was significantly influenced by mass-transport

processes. Slope instability on the flanks of tectonically active

structures generated in many places an oversteepened morph-

ology that failed, triggering gravity-driven deep-water mass

transport ranging from sliding and slumping to the generation of

high-energy, sediment-laden turbulent density flows.

(2) The detailed analysis of one major intra-chalk channel in

the north of the study area documents for the first time the

subsurface presence of a kilometre-scale, sinuous, leveed sedi-

ment-transport system that displays many of the architectural

elements known from siliciclastic deep-water turbidite systems.

This channel seems to have formed an important conduit for

recurrent turbulent flows that transported failed chalk material

into the deeper chalk depocentres.

(3) At the same stratigraphic level, there are other channels

that contrast with the northern channel in that they display

typical elements of contourite systems that were eroded by

powerful bottom currents and filled laterally by chalk drifts. If

integrated with the above conclusions, it seems likely that a

Fig. 11. Summary diagram of the depositional systems interpreted from seismic reflection discontinuities at different stratigraphic levels in the North Sea

Chalk Group offshore Denmark. (a) The Chalk I level is characterized by a generally high reflection continuity with only few seismic dissemblance

features interpreted as isolated channels or small-scale mass-transport deposits. (b) Reflection discontinuity reaches a maximum at Chalk II level.

Depositional systems interpreted at this level include (1) kilometre-scale slumps and slides located alongside tectonically active inversion ridges;

(2) sinuous, leveed turbidite channels; (3) incisions interpreted as bottom-current channels filled laterally by drift deposits. (c) Seismic reflection

discontinuity wanes in the Chalk III interval. Ridge areas exhibit a dendritic to circular, vertically mounded dissemblance facies interpreted to indicate

minor mass movement (slumping, sliding, creep?), surface karst (sensu Huuse 1999) or hypogenic karst (sensu Palmer 1991).

S . BACK ET AL .404

combination of active inversion tectonics, gravity-driven pro-

cesses and a superimposed bottom-current regime ultimately

controlled the reworking, redistribution and redeposition of the

Chalk Group strata in the North Sea Central Graben, which

makes generalized predictions of deposit type and its associated

reservoir properties difficult.

The authors thank Maersk Olie og Gas, Shell EPE Geological Services

and the DUC partners for initiating and supporting this study. The

contributions of many of their staff technically and/or by discussion is

particularly acknowledged. Personally thanked are T. Fronval, A. Uldall,

A. Wetzelaer and R. Kelly for their project input. The corrections,

comments and suggestions by reviewers M. Huuse, D. Evans and GSL

editor G. Hampson significantly improved an earlier version of the

manuscript, and their contribution to the final paper is highly appreciated.

Schlumberger is thanked for providing ‘PETREL’ under an Academic

User License Agreement. Seismic Micro-Technology is thanked for

providing ‘KINGDOM Software’ under an Educational User License

Agreement. This study is a spin-off of project DINGO (RE 2697/3-1)

funded by the Deutsche Forschungsgemeinschaft (DFG).

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Received 29 March 2010; revised typescript accepted 2 August 2010.

Scientific editing by Gary Hampson.

3D SEISMIC GEOMORPHOLOGY IN THE CHALK GROUP 405

The Geological Society of London, Burlington HouseLaunch of the NERC Long-term Co-evolution of Life and the PlanetProgramme

Convenors:

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Keynote speakers include:

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The Earth that sustains us today was born out of a few remarkablerevolutions, started by evolutionary innovations and marked by globalenvironmental consequences, including abrupt rises in oxygen andextreme glaciations. The coupled evolution of life and the planet hascontinued up to the present, and now includes the planet-reshapingactivities of our species.

This two-day discussion meeting will showcase recent progress inunderstanding the development of the Earth as a system. The meeting will outline how new science can help in tying down critical uncertainties,regarding the nature and timing of past events. Finally, it will explore how an improved understanding of life and the planet in the past can help us achieve future sustainability.

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